Some people have got them to work well in some instances, but like the poster said they need finishing and fettling. A bit like a BSA SuperSport.
The problem comes with them when people take the untrimmed, unprepped spring and cram it into some unsuspecting medium power rifle.
That's not to say you should put one in your Theoben. That would be like walking around with someone elses dirty underpants on. It's an Ox and should stay in the Box has been the BBS mantra for the last 18 years.